1. Field of the Invention
The present invention relates to low bleed silicone greases and to methods for their production. Specifically, this invention relates to silicone grease compositions which maintain lubricity while exhibiting little or no oil bleed even upon long term standing at elevated temperatures and/or environmental exposure. Grease compositions of the present invention maintain lubricity even when in contact with various materials, including copper, aluminum, other metals, elastomers, thermoplastics and ceramics and even in the presence of a low or high voltage electrical fields.
2. Background of the Invention and Related Information
Many silicone greases physically separate allowing the organopolysiloxane "oil" to bleed away from the site of lubrication over time. This is because many silicone greases contain a relatively mobile organopolysiloxane material which is physically thickened with a finely divided mineral filler to give the product a grease-like consistency. However, the organopolysiloxane retains some mobility to a degree which is dependent upon its viscosity and often will eventually bleed away from the site of lubrication.
Oil bleed is a troublesome problem when such greases are exposed to field aging conditions. As the oil bleeds away from the site of lubrication, the physical barrier between mated parts can become compromised and direct contact may result. In some cases, such direct contact of interfaces which are meant to stay lubricated in the field, results in adhesion across the interface which inhibits separation of the parts. In addition, the mineral thickening material which is left behind after the oil bleeds away, acts as an abrasive which further impedes separation.
In certain high voltage applications, such as with loadbreak elbows, separation failure endangers equipment and personnel. Furthermore, in such accessories, oil bleed off may also result in voltage stresses which exceed design limitations resulting in field failures in the absence of a separation attempt.
The present invention was developed as a result of extensive investigation into the failure mechanisms of various silicone greases for the purpose of solving the oil bleed problem associated with prior art grease compositions. For example, prior art grease compositions contain organopolysiloxanes in the viscosity range of 800 centistokes (cS) to 3500 cS at 25.degree. C. which is too low to be adequately physically stable for many applications. Greases prepared from such oils require very high mineral thickener levels, such as up to 60% w/w to form a grease-like material. As oil bleed occurs, a high level of abrasive mineral thickener is left behind at the previously lubricated interface.